(2)  
This switch starts the alternate extension cycle. When the switch is moved to DN, electrical power is supplied to the alternate extend actuator.

 B.
Power Pack (Fig. 2)

__________
 (1)  
The power pack is an electrically-operated hydraulic pump. The power pack supplies a specified volume of hydraulic fluid from the left system reservoir to the actuators and valves in the alternate extension system. The power pack is installed on the keel beam in the right wheel well for the main landing gear. The location is forward of the door actuator for the main landing gear.

 C.
Door Lock Release Actuator (Fig. 3 and 4)

__________________________
 (1)  The door lock release actuator is a piston-type actuator that operates hydraulically in one direction. An internal valve sends hydraulic fluid out of the actuator when the piston is fully extended. The valve is spring-loaded to go back to the normally retracted position when hydraulic pressure is removed. The actuator supplies the force to mechanically release the internal lock in the door actuator.
POWER
PACK

SEE  A
RIGHT MAIN LANDING GEAR WHEEL WELL  HYDRAULIC PUMP 
KEEL BEAM 
INBD  FWD  A 
Alternate Extension System Power Pack Figure 2 

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A

757
MAINTENANCE MANUAL

SEE A
MAIN LANDING GEAR WHEEL WELLS (KEEL BEAM)
DOOR ACTUATOR
FWD
A
Main Landing Gear Door Lock Release Mechanism Figure 3
DOOR SAFETY VALVE
DOOR RELEASE INTERLOCK ACTUATOR
DOOR LOCK RELEASE ACTUATOR

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A

757
MAINTENANCE MANUAL

SEE  A 
FWD 
NOSE LANDING GEAR WHEEL WELL  DOOR LOCK 
(LEFT WALL)  ACTUATOR RELEASE 

SEE B
A
SEE C
NOSE LANDING GEAR WHEEL WELL
(FORWARD WALL)

DOOR RELEASE INTERLOCK ACTUATOR
NOSE LANDING GEAR DOOR SAFETY VALVE
CB
Nose Landing Gear Door Lock Release Mechanism Figure 4

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A

757
MAINTENANCE MANUAL

 (2)  
A door lock release actuator is supplied for each landing gear. Each actuator for the main landing gear is installed immediately aft of the door actuator, on the keel beam in the wheel well for the main landing gear. The actuator for the nose landing gear is installed on the left wall of the wheel well for the nose landing gear.

 D.
Door Safety Valve (Fig. 3 and 4)

_________________
 (1)  
The door safety valve is a two-position valve which is installed in the door close line. It makes sure the landing gear door does not accidentally close. The valve permits free flow of hydraulic fluid to the door actuator during normal extension. During alternate extension, the valve is mechanically set in position and locked to do the functions that follow:

 (a)  
Prevent hydraulic flow through the door close line that goes from the sequence valve to the door actuator

 (b)
Open the line from the door actuator retract port to return. The valve is set in this position to make sure that no hydraulic fluid will be caught in the hydraulic line to cause resistance when you try to open the door. It also prevents supply of hydraulic pressure to the door actuator to arm the door to close without the door actually moved to the closed position.

 (2)  
There is one door safety valve for each landing gear. Each main landing gear valve is installed on the keel beam in the wheel well for the main landing gear. The valve can be found immediately aft of the door actuator. The nose landing gear valve is installed near the top of the forward wall of the wheel well for the nose landing gear.

 E.
Door Release Interlock Actuator (Fig. 3 and 4)

_______________________________
 (1)  
The door release interlock actuator has a hydraulic piston-type actuator and a solenoid valve assembly. This assembly releases the lock from the door safety valve to permit the landing gear door to be closed. To operate, an electrical input to the assembly and normal hydraulic pressure are necessary. The electrical signal causes the solenoid valve to move and supply hydraulic pressure to the actuator. The actuator supplies the force to change the position of the door safety valve to remove the lock.

 (2)  
There is one door release interlock actuator for each landing gear. Each main landing gear actuator is installed immediately aft of the door actuator, on the keel beam in the wheel well for the main landing gear. The nose landing gear actuator is installed at the forward end of the left wall of the wheel well for the nose landing gear.
　
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本文鏈接地址：757 AMM 飛機維護手冊 起落架 CHAPTER 32 - LANDING GEAR 2(90)